A new complementing cell line for replication-incompetent E1-deleted adenovirus propagation

Adenoviral Gene Therapy Vectors in Clinical Use-Basic Aspects with a Special Reference to Replication-Competent Adenovirus Formation and Its Impact on Clinical Safety

Adenoviral vectors are commonly used in clinical gene therapy. Apart from oncolytic adenoviruses, vector replication is highly undesired as it may pose a safety risk for the treated patient. Thus, careful monitoring for the formation of replication-competent adenoviruses (RCA) during vector manufacturing is required. To render adenoviruses replication deficient, their genomic E1 region is deleted. However, it has been known for a long time that during their propagation, some viruses will regain their replication capability by recombination in production cells, most commonly HEK293. Recently developed RCA assays have revealed that many clinical batches contain more RCA than previously assumed and allowed by regulatory authorities. The clinical significance of the higher RCA content has yet to be thoroughly evaluated. In this review, we summarize the biology of adenovirus vectors, their manufacturing methods, and the origins of RCA formed during HEK293-based vector production. Lastly, we share our experience using minimally RCA-positive serotype 5 adenoviral vectors based on observations from our clinical cardiovascular gene therapy studies.

A Method to Generate and Rescue Recombinant Adenovirus Devoid of Replication-Competent Particles in Animal-Origin-Free Culture Medium

Adenoviruses are promising vectors for vaccine production and gene therapy. Despite all the efforts in removing animal-derived components such as fetal bovine serum (FBS) during the production of adenovirus vector (AdV), FBS is still frequently employed in the early stages of production. Conventionally, first-generation AdVs (E1 deleted) are generated in different variants of adherent HEK293 cells, and plaque purification (if needed) is performed in adherent cell lines in the presence of FBS. In this study, we generated an AdV stock in SF-BMAdR (A549 cells adapted to suspension culture in serum-free medium). We also developed a limiting dilution method using the same cell line to replace the plaque purification assay. By combining these two technologies, we were able to completely remove the need for FBS from the process of generating and producing AdVs. In addition, we demonstrated that the purified AdV stock is free of any replication-competent adenovirus (RCA). Furthermore, we demonstrated that our limiting dilution method could effectively rescue an AdV from a stock that is highly contaminated with RCA.

Protective immunity against tularemia provided by an adenovirus-vectored vaccine expressing Tul4 of Francisella tularensis

Francisella tularensis, a category A bioterrorism agent, is a highly infectious organism that is passed on via skin contact and inhalation routes. A live attenuated vaccine strain (LVS) has been developed, but it has not been licensed for public use by the FDA due to safety concerns. Thus, there exists a need for a safer and improved vaccine. In this study, we have constructed a replication-incompetent adenovirus, Ad/opt-Tul4, carrying a codon-optimized gene for expression of a membrane protein, Tul4, of F. tularensis LVS. Its ability to protect against lethal challenge and its immunogenicity were evaluated in a murine model. An intramuscular injection of a single dose (1 × 10(7) PFU) of Ad/opt-Tul4 elicited a robust Tul4-specific antibody response. Assays suggest a Th1-driven response. A single dose elicited 20% protection against challenge with 100 × 50% lethal dose (LD(50)) F. tularensis LVS; two additional booster shots resulted in 60% protection. In comparison, three doses of 5 μg recombinant Tul4 protein did not elicit significant protection against challenge. Therefore, the Ad/opt-Tul4 vaccine was more effective than the protein vaccine, and protection was dose dependent. Compared to LVS, the protection rate is lower, but an adenovirus-vectored vaccine may be more attractive due to its enhanced safety profile and mucosal route of delivery. Furthermore, simple genetic modification of the vaccine may potentially produce antibodies protective against a fully virulent strain of F. tularensis. Our data support the development and further research of an adenovirus-vectored vaccine against Tul4 of F. tularensis LVS.

Development of a recombinant adenovirus vector production system free of replication-competent adenovirus by utilizing a packaging size limit of the viral genome

In a conventional adenovirus (Ad) vector production method using 293 cells, homologous recombination between Ad vector DNA and 293 cell-derived Ad E1 DNA occurs with low efficiency, resulting in the generation of replication-competent adenovirus (RCA). RCA can induce the spread of replication-incompetent Ad vectors, leading to unexpected tissue damage. In order to overcome this problem, we developed an Ad vector production system free of RCA generation by utilizing the Ad packaging size limit of the viral genome. It is well known that up to approximately 105% (37.7 kb) of the wild-type genome (35.9 kb) can be packaged in the Ad virion. We designed the Ad vector genome by insertion of a transgene expression cassette into the E3 region, such that homologous recombination between the Ad vector DNA and 293 cell-derived Ad E1 DNA would produce an Ad vector genome that exceeds in the size of the packaging limit. In accord with our strategy, no RCA generation was observed during the passages when we used the E1 (3.2kb)-deleted Ad vectors containing a more than 3.0-kb transgene expression cassette in the E3 region. In contrast, the E1 (3.2kb)-deleted Ad vectors, which retain 37.7 kb of the viral genome and have an insertion of a 2.1-kb transgene expression cassette in the E3 region, generated RCA, although RCA derived from this Ad vector exceeded the packaging size limit (105.0%). These results suggest that RCA generation can be avoided when the genome size of RCA is more than 108.3% (38.9 kb) of the wild-type Ad genome. This Ad vector production system generates safe, easy, and efficient Ad vector stock for both basic study as well as clinical research.

Host cells and cell banking

Gene therapy based on the use of viral vectors is entirely dependent on the use of animal cell lines, mainly of mammalian origin, but also of insect origin. As for any biotechnology product for clinical use, viral -vectors have to be produced with cells derived from an extensively characterized cell bank to maintain the appropriate standard for assuring the lowest risk for the patients to be treated. Although many different cell types and lines have been used for the production of viral vectors, HEK293 cells or their derivatives have been extensively used for production of different vector types: adenovirus, oncorectrovirus, lentivirus, and AAV vectors, because of their easy handling and the possibility to grow them adherently in serum-containing medium as well as in suspension in serum-free culture medium. Despite this, these cells are not necessarily the best for the production of a given viral vector, and there are many other cell lines with significant advantages including superior growth and/or production characteristics, which have been tested and also used for the production of clinical vector batches. This chapter presents basic -considerations concerning the characterization of cell banks, in the first part, and, in the second part, practically all cell lines (at least when public information was available) established and developed for the production of the most important viral vectors (adenoviral, oncoretroviral, lentiviral, AAV, baculovirus).

Adenoviral producer cells

Adenovirus (Ad) vectors, in particular those of the serotype 5, are highly attractive for a wide range of gene therapy, vaccine and virotherapy applications (as discussed in further detail in this issue). Wild type Ad5 virus can replicate in numerous tissue types but to use Ad vectors for therapeutic purposes the viral genome requires modification. In particular, if the viral genome is modified in such a way that the viral life cycle is interfered with, a specific producer cell line is required to provide trans-complementation to overcome the modification and allow viral production. This can occur in two ways; use of a producer cell line that contains specific adenoviral sequences incorporated into the cell genome to trans-complement, or use of a producer cell line that naturally complements for the modified Ad vector genome. This review concentrates on producer cell lines that complement non-replicating adenoviral vectors, starting with the historical HEK293 cell line developed in 1977 for first generation Ad vectors. In addition the problem of replication-competent adenovirus (RCA) contamination in viral preparations from HEK293 cells is addressed leading to the development of alternate cell lines. Furthermore novel cell lines for more complex Ad vectors and alternate serotype Ad vectors are discussed.

Mucosal vaccination with a multicomponent adenovirus-vectored vaccine protects against Streptococcus pneumoniae infection in the lung

Streptococcus pneumoniae is a major bacterial respiratory pathogen. Current licensed pneumococcal polysaccharide and polysaccharide-protein conjugate vaccines are administered by an intramuscular injection. In order to develop a new-generation vaccine that can be administered in a needle-free mucosal manner, we have constructed early 1 and 3 gene regions (E1/E3) deleted, replication-defective adenoviral vectors encoding pneumococcal surface antigen A (PsaA), the N-fragment of pneumococcal surface protein A (N-PspA), and the detoxified mutant pneumolysin (PdB) from S. pneumoniae strain D39. Intranasal vaccination with the three adenoviral vectors (Ad/PsaA, Ad/N-PspA, and Ad/PdB) in mice resulted in robust antigen-specific serum immunoglobulin G responses, as demonstrated by an enzyme-linked immunosorbent assay. In addition, nasal mucosal vaccination with the combination of the three adenoviral vectors conferred protection against S. pneumoniae strain D39 colonization in mouse lungs. Taken together, these data demonstrate the feasibility of developing a mucosal vaccine against S. pneumoniae using recombinant adenoviruses for antigen delivery.

An adenoviral vector-based mucosal vaccine is effective in protection against botulism

A replication-incompetent adenoviral vector encoding the heavy chain C-fragment (H_C50) of botulinum neurotoxin type C (BoNT/C) was evaluated as a mucosal vaccine against botulism in a mouse model. Single intranasal inoculation of the adenoviral vector elicited a high level of H_C50-specific IgG, IgG1 and IgG2a in sera and IgA in mucosal secretions as early as 2 weeks after vaccination. The antigen-specific serum antibodies were maintained at a high level at least until the 27th week. Immune sera showed high potency in neutralizing BoNT/C as indicated by in vitro toxin neutralization assay. The mice receiving single dose of 2 × 10⁷ p.f.u. (plaque-forming unit) of adenoviral vector were completely protected against challenge with up to 10⁴ × MLD₅₀ of BoNT/C. The protective immunity showed vaccine dose dependence from 10⁵ to 2 × 10⁷ p.f.u. of adenoviral vector. In addition, animals receiving single intranasal dose of 2 × 10⁷ p.f.u. adenoviral vector could be protected against 100 × MLD₅₀ 27 weeks after vaccination. Animals with preexisting immunity to adenovirus could also be vaccinated intranasally and protected against lethal challenge with BoNT/C. These results suggest that the adenoviral vector is a highly effective gene-based mucosal vaccine against botulism.

From the first to the third generation adenoviral vector: what parameters are governing the production yield?

Human adenoviral viral vector serotype 5 (AdV) is presently the primary viral vector used in gene therapy trials. Advancements in AdV process development directly contribute to the clinical application and commercialization of the AdV gene delivery technology. Notably, the development of AdV production in suspension culture has driven the increase in AdV volumetric and specific productivity, therefore providing large quantities of AdV required for clinical studies. This review focuses on detailing the viral, cell and cell culture parameters governing the productivity of the three generations of AdV vectors.

Protective immunity against botulism provided by a single dose vaccination with an adenovirus-vectored vaccine

Botulinum neurotoxins cause botulism, a neuroparalytic disease in humans and animals. We constructed a replication-incompetent adenovirus encoding a synthesized codon-optimized gene for expression of the heavy chain C-fragment (H(C)50) of botulinum neurotoxin type C (BoNT/C). This recombinant human serotype 5 adenoviral vector (Ad5) was evaluated as a genetic vaccine candidate against botulism caused by BoNT/C in a mouse model. A one-time intramuscular injection with 10(5) to 2 x 10(7)pfu of adenoviral vectors elicited robust serum antibody responses against H(C)50 of BoNT/C as assessed by ELISA. Immune sera showed high potency in neutralizing the active BoNT/C in vitro. After a single dose of 2 x 10(7)pfu adenoviral vectors, the animals were completely protected against intraperitoneal challenge with 100 x MLD(50) of active BoNT/C. The protective immunity appeared to be vaccine dose-dependent. The anti-toxin protective immunity could last for at least 7 months without a booster injection. In addition, we observed that pre-existing immunity to the wild-type Ad5 in the host had no significant influence on the protective efficacy of vaccination. The data suggest that an adenovirus-vectored genetic vaccine is a highly efficient prophylaxis candidate against botulism.